1. Field of the invention
The present invention relates to an image forming apparatus. More specifically, the present invention relates to an image forming apparatus such as a copying machine, printer, facsimile and etc. in which a photosensitive member is exposed with a light image to form an electrostatic latent image which is then developed by a toner.
2. Description of the Prior Art
In a kind of an electrophotographic apparatus, electrophotographic processes such as charging, exposure, developing, transferring, separating, cleaning, erasing, and etc. are repeated around a photosensitive drum. Not only does the sensitivity of the photosensitive drum become low but also a residual voltage (Vr) increases due to (1) fatigue of the photosensitive drum by an exposure light and an erasing light, (2) deterioration of a photosensitive layer by ozone which is generated in charging, transferring and separating, (3) decrease of the thickness of the photosensitive layer by friction with a cleaning blade, and etc. Resultingly, a voltage of a portion corresponding to a white ground of an original, that is, an exposure voltage (VL) which was approximately equal to zero in an initial state increases. Therefore, a so-called fogging phenomenon occurs, and accordingly, the life of the photosensitive member decreases.
Therefore, in the past, it was known to use an electrophotographic apparatus in which a developing bias voltage (Vb) is increased for prolonging the life of the photosensitive member (Japanese Patent Application Laying-Open No. 50-46334), and an electrophotographic apparatus in which the amount of light from the exposure lamp is increased for prolonging the life of the photosensitive member.
In the former apparatus, there was a problem that the image density is lowered because a voltage difference between a dark voltage (Vd) corresponding to a black portion of the original and the developing bias voltage (Vb) becomes small. On the other hand, in the latter apparatus, it was impossible to deal with the increase of the exposure voltage (VL) according to the increase of the residual voltage (Vr), and therefore, the latter apparatus had its limit.